Coaxial cable transition arrangement

ABSTRACT

An arrangement for transferring high frequency microwave signals between a cable and a microstrip printed circuit on a dielectric substrate is disclosed wherein a first microstrip track on a first substrate reactively couples with a second microstrip track on a second substrate, which second substrate is connected to an inner conductor of a coaxial cable and a ground plane associated with the first microstrip track is connected to the ground shielding of the coaxial cable. Since the microwave signals are reactively coupled by means of printed circuit tracks on a first dielectric substrate to printed circuit tracks on a second dielectric substrate, a non-contacting RF connection is established. This avoids the potential formation of intermodulation products which occur in metal--metal (galvanic) junctions. A further, advantage arises in that a d.c. block is automatically incorporated within the arrangement, reducing the need for separate coupled lines, capacitors and the like. A method of transferring high frequency microwave signals between a cable and a printed circuit on a dielectric substrate is also disclosed.

RELATED APPLICATION

This application is related by disclosure to U.S. application, Ser. No.08/832,408, also assigned to Nortel Networks Corporation.

BACKGROUND OF THE INVENTION

This invention relates to a coaxial cable transition to a planarsubstrate arrangement, such as a coaxial cable to microstriparrangement.

Coaxial cable is widely employed in system configuration, wheremicrowave and radio signals are processed. A typical use of a coaxial toplanar substrate transition is in a mobile communications network basestation where receive and transmit electronics are connected to atriplate or layered antenna by way of a coaxial cable. One form oftriplate antenna comprises a microstrip feed network printed on adielectric film or substrate which provides the feed probes or patcheswhich extend into or are arranged within radiating apertures definedthrough the outermost groundplane of the triplate antenna. In such anarrangement, the central conductor of a coaxial cable is soldereddirectly to the microstrip circuit of the antenna. The axis of thecentral conductor can either be in-line or orthogonal with respect tothe substrate and the earthed sheath is connected to the groundplanes ofthe antenna. Alternatively, the microstrip array may be formed upon aprinted circuit board manufactured from a substance such as PTFE. U.S.Pat. No. 4,918,458 (Ford Aerospace) describes such an antennaarrangement which is fed by way of a coaxial supply cable. GB-A-2007919(Raytheon) also provides an antenna arrangement which is fed by way of acoaxial supply cable.

These types of configuration, while easy to manufacture can suffer fromthe generation of passive intermodulation products. Power handlingcapabilities can be limited since high losses will result from theisolating distances necessary from the coaxial transition section to anypower dividers such as Wilkinson couplers. Further problems arise in theuse of the dielectrics having high temperature capabilities necessary inorder to allow solder connections to be made. Coupled lines can bepresent in order to provide a d.c. block in cases such as activeantennas.

In the design of mechanical connections with microwave conductors,extreme care needs to be exercised for critical applications requiringhigh linearity, for example, cellular radiocommunications and satellitecommunications. In the case where components are welded or soldered,attention needs to be paid to the electrical conductor's surface;irregularities and imperfect metal to metal contacts lead to electricalnon-linearities. This introduces passive intermodulation, in whichdeleterious, spurious signals are generated and, generally, theseeffects vary with frequency, contact pressure, age and other factors.

An object of the invention is to provide an improved coaxial cable tomicrostripline connection with high mean or peak power handling and verylow passive intermodulation product generation.

SUMMARY OF THE INVENTION

In accordance with one aspect of the invention, there is provided anarrangement for transferring high frequency microwave signals between acable and a microstrip printed circuit on a dielectric substrate, thearrangement comprising: a coaxial cable, a dielectric substrate carryinga first microstrip printed circuit and an intermediate dielectricsubstrate carrying a second printed circuit; wherein an inner conductorof the coaxial cable is connected to the second printed circuit of theintermediate dielectric substrate and the outer conductor of the coaxialcable is connected to a ground plane associated with said dielectricsubstrate; and wherein the printed circuit on the intermediatedielectric is operable to reactively couple all the signals from thecoaxial cable to the first printed circuit on said dielectric substratethrough the printed circuit of the intermediate dielectric substrate.

The intermediate dielectric substrate can carry a metallized surfaceacting as a ground plane and to which the ground of the coaxial cable isconnected, which ground plane can reactively couple with said groundplane associated with said dielectric substrate.

The inner conductor of the coaxial cable port can be connected to afirst node of a five port rat-race-coupler with each of the two nodesadjacent the first node feeding in a balanced fashion an output linewhich is operable to couple with a printed circuit on said dielectric.The other two nodes of the rat-race-coupler can be connected to groundby terminating resistors.

The other two output nodes of the rat-race-coupler can be connected tothe two output arms of a Wilkinson coupler whereby a single transmissionline output from the coaxial cable is established. Alternatively, theother two output nodes of the rat-race-coupler can be each connected toa Wilkinson coupler whereby four transmission line outputs from thecoaxial cable are established.

The dielectric substrate of the intermediate board can be manufacturedfrom PTFE; said dielectric substrate can be a polyester film. Theprinted circuit can be arranged in the form of microstrip.

In accordance with a still further aspect of the invention, there isprovided a method of transferring high frequency microwave signalsbetween a cable and a printed circuit on a dielectric substrate, in anarrangement comprising: a coaxial cable, a dielectric substrate carryinga first printed circuit and an intermediate dielectric substratecarrying a printed circuit; the method comprising the steps of:

transferring said signals through an inner conductor of the coaxialcable to the printed circuit of the intermediate dielectric substrate,the outer conductor of the coaxial cable being connected to a groundplane associated with said dielectric substrate; and

reactively coupling all signals between the coaxial cable and the firstprinted circuit on said dielectric substrate through the printed circuitof the intermediate dielectric substrate.

In accordance with a further aspect of the invention, there is provideda coaxial to planar substrate coupling arrangement wherein a firstmicrostrip track on a first substrate reactively couples with a secondmicrostrip track on a second substrate, which second substrate isconnected to an inner conductor of a coaxial cable and a ground planeassociated with the first microstrip track is connected to the groundshielding of the coaxial cable.

The microwave signals are reactively coupled by means of printed circuittracks on a first dielectric substrate to printed circuit tracks on saiddielectric substrate, whereby a non-contacting RF connection isestablished. This avoids the potential formation of intermodulationproducts which occur in metal--metal (galvanic) junctions.

Further, a d.c. block is automatically incorporated within thearrangement, reducing the need for separate coupled lines, capacitorsand the like. D.C./low frequency blocks are useful--and indeednecessary--for isolating wanted signal components from other signalscarrying, for example, unwanted d.c or lower frequency bias, digital orother signals. The incorporation of a reactively coupled groundplane hasthe advantage that it can facilitate the avoidance of inconsistenciessuch as multiple ground returns (ground loops).

By having the inner conductor of the coaxial cable connected to amicrostrip circuit separate from the first microstrip track, thedielectric substrate supporting the first microstrip track/feed networkneed not be manufactured from a high temperature dielectric. That is tosay the dielectric can be a thin film, for example 0.075 mm thick, witha microstrip circuit printed thereon. This allows the use of a cheapdielectric such as a low temperature polyester film.

Preferably the microstrip is arranged in a triplate configuration toreduce losses, but microstrip transmission lines without a second groundplane, as in the case of triplate, may be used. The microstrip linesfrom the solder connection on the second dielectric board, thetransition board, may separate into two in-phase, oppositely directedmicrostrip lines or may form a node of a balanced five node rat-racecircuit element with power being coupled from the two nodes adjacent theinput node. It has been found that a balanced five node device providesa convenient coupling arrangement, but other types of rat-race or othercombiner/splitter are possible. Preferably microstrip elements arearranged around the input node to suppress propagation of undesiredmodes having significant field components parallel with the groundplanes.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings, in which:

FIG. 1 shows a first embodiment of the invention;

FIG. 2 details the first embodiment in partial section;

FIG. 3 shows the relative positions of coupled portions;

FIG. 4 shows a first coaxial termination element;

FIG. 5 shows a second coaxial termination element;

FIG. 6 shows a rat-race-coupling arrangement; and

FIGS. 7 and 8 demonstrate the equivalence of the embodiments.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring now to FIG. 1, there is shown a first arrangement inaccordance with the invention wherein a coaxial cable 10 having an outerconductor 11 and having a ground connection transition body 12 isattached to a first ground plane 14 of the triplate structure. Anexample of a transition body is shown in detail in FIG. 4. The innerconductor 16 of the coaxial cable is connected to a transitionaldielectric substrate 18 having a microstrip network printed thereon,arranged in a `T` layout on the surface opposite the first ground plane14, which does not detail mounting features for transition body 12. Athin dielectric 20 supports a microstrip network for the triplatestructure. The dielectric 20 has a cut-out portion corresponding to thearea of the solder joint 22 effected on the transition portion 18 fromthe inner conductor of the coaxial cable. The microstrip network isprinted on the side of the dielectric facing away from the firstgroundplane 14. Dielectric layers 21,23 such as foam layers 24,26 areplaced either side of dielectric 20, around the transition board 18 andaround the optional secondary transition board 30. Optional transitionboard 30 serves to prevent the solder from contacting with a secondground plane 32. The microstrip patch elements 34, 36 of the transitionboard 18 capacitively couple with microstrip elements 38, 40 of themicrostrip network on dielectric 20.

FIG. 2 details the sections of the embodiment shown in FIG. 1, but doesnot detail coaxial cable 10 and transition body 12. The triplatestructure is defined by two metal plates 14, 32 made from, for example,aluminium alloy. A dielectric film 20 supports a microstrip network,which film is supported between two layers of high density foam 21, 23,24, 26 (FIG. 1) whereby optimum distances between the film 20 and themetallic plates of the triplate structure are maintained. Theintermediate boards of the transition arrangement 18, 30 lie either sideof the dielectric film 20, while a plastic sheet such as polyester 33isolates the ground plane 15 on the underside of the intermediate board18 from the ground plane 14 of the triplate structure and the groundingeffect is thus reactively coupled.

FIG. 3 details, in a perspective spaced-apart relationship, theintermediate boards 18, 30 of the transition arrangement. The dielectricfilm 20 having a metallized track with a coupling patch 40 on a firstside is positioned with its second side against the intermediate board18. Coupling patch 40 is arranged opposite a similarly shaped metallizedpatch 36 of the microstrip network on the intermediate board 18 toensure optimum coupling--although the coupling region may in fact be nomore than a portion of metallized line. Conveniently, the microstripline from the coaxial cable divides into two probes, which probesseparately couple with corresponding patches on the polyester film sincethe power can be easily split between the two arms without excessivepower loss due to reflections. Alternatively, the two arms from thecoaxial feed point can feed a Wilkinson divider, whereby four couplingpatches may couple with corresponding patches on the polyester film.

One form of coaxial termination is shown in FIG. 4, and depicts therelative positions, albeit not to scale, of coupled portions of afurther embodiment, in the region where the intermediate board portionsoverlap. In this example, a connector-socket 12 is positioned within arecess of groundplane 14. Drilled and tapped holes 11 are arranged toaccept bolts (not shown) which fasten the arrangement to a triplatestructure 14,18,30 & 32. Alternatively, the bolts may be self tapping. Afemale contact 19 is soldered to the board and to the microstrip tracks.This contact has a split sleeve configuration which can engage a centralconductor of a coaxial cable in a sliding contact fashion, which canaccomodate movement due to thermal expansion and other effects. Thecentral portion of the connector has a recess which is internallythreaded at the entrance and an abutment portion, the abutment portionbeing shaped to abut against a ferrule associated with the end of acoaxial cable upon connection of screw-threaded bolt 13.

FIG. 5 shows a second type of coaxial cable to stripline/microstripconfiguration having bolts 81 which attach the connector to thedielectric structure 82 (which can be flexible). Details of the specificconnection between the inner conductor of the coaxial cable 86 and thesubstrate 82 are not shown. The abutment portion 84 has acircumferential line or edge contact arrangement 80, which edge iscompressed upon abutment with the other ferrule or abutment portion. Theferrule 85 could possess the circumferential line or edge contactarrangement. The ferrule 85 indicates a coaxial connection, referencenumeral 83 indicates part of the connector structure on the other sideof the dielectric to the abutment portions 84, 85 and to which theabutment portions 84, 85 are connected by means of bolts 81.

FIG. 6 details a second type of microstrip network for the transitionsection 18, comprising a balanced five port rat-race circuit element 50,wherein one of the nodes 52 of the rat-race is the coaxial-soldertransition. The nodes or ports 54, 56 either side of the input node 52act as output ports which can feed couplers such as Wilkinson couplers(not shown) which enable power to be divided or combined with respect tothe output arms. Thus, using two Wilkinson couplers, four coupledportions can be provided from the arrangement. This is a compactcoupling arrangement, which is especially useful in microstrip antennaarrangements. Metallized portions 70, 72 act to confine the microwavepropagation along the rat-race rather than between the microstrip linesand the ground plane in a parasitic and lossy fashion. Terminatingresistors R1, R2 are preferably placed at the unused ports of therat-race, as is well known. A grounded area can be provided on the sameside as the microstrip pattern to aid parasitic mode suppression. Such agrounded area can be readily fabricated by appropriate metallization andextending vias from the earth plane on the other side of theintermediate board, and/or by metallizing around the edge of thesubstrate.

FIGS. 7 and 8 show the equivalence of the two forms of couplingarrangements as shown in FIGS. 1 and 6. The rat-race is internallymatched to reduce losses and by having an in-phase splitter, the portsare in-phase. The metallized portion 70 is preferably connected to therat-race by a resistive element to avoid over-moding. Note also thatinstead of feeding two Wilkinson couplers, the two ports from therat-race could feed the two input arms of a Wilkinson coupler to providea single output.

By providing a reactively coupled connection, direct contact betweendissimilar metals is reduced, thus reducing a source of inter-modulationnoise and non-linearities. Preferably, through the use of silver platedcomponents, fluxless solder and the use of solder reflow techniqueswhere appropriate, noise generation is further reduced.

In order to keep manufacturing costs to a minimum the transition bodycan be a simple turned part and incorporate a slot in the mating face.This slot can allow self tapping screws to be used to fasten thetransition body to the transition board assembly. This feature has twoadvantages: firstly, alignment is only necessary in one coordinatedirection between the fixing holes in the transition board assembly andthe transition body, and secondly, the transition body is cheap tomanufacture as it avoids the need for costly tapped holes for fixingscrews.

The female contact soldered to the transition board allows the centerconductor of the semi-rigid cable to slide within it thus avoidingmechanical stress during thermal expansion of the cable and the use ofexisting well proven connector parts within the transition assures verylow intermodulation product generation. The microstrip networks can beformed from copper and the substrate upon which the microstrip networksare supported can be polyester, both of which being commonly used forsuch purposes.

The transition board is preferably manufactured from PTFE, which whenmetallized can provide a solderable substrate for the female contact inthe transition. PTFE has a relatively high melting point which lendsitself readily to soldering. The use of PTFE is preferable to that of afoam/film/foam sandwich for triplate since the PTFE can betteraccomodate high powers, is of low loss and, further, PTFE exhibits abetter thermal conductivity than foam/film/foam. The assembly can thushandle relatively high powers and operate within an acceptabletemperature range.

The coaxial cable may be rigid, semi-rigid or flexible. The groundplanes shown may be formed from aluminium alloy, which offers a goodstrength to weight ratio and is highly corrosion resistant.

We claim:
 1. An arrangement for transferring high frequency microwavesignals between a cable and a printed circuit on a dielectric substrate,the arrangement comprising: a coaxial cable (10) having an innerconductor (16) and an outer conductor (11), a first around plane (14), afirst dielectric substrate (20) having a first printed circuit, anintermediate dielectric substrate (18) having a second printed circuitand a ground plane (15);wherein the second printed circuit comprises afive port rat-race coupler and a Wilkinson coupler having two couplingarms and an output; wherein the inner conductor of the coaxial cable isconnected to a first port of said five port rat-race-coupler, whereinthird and fourth ports of said rat-race-coupler are connected to groundby terminating resistors (R1, R2); and wherein the second and fifthports (54, 56) provide output ports which are connected to the twocoupling arms of the Wilkinson coupler which is operable to provide acoupled output port; wherein said outer conductor of the coaxial cable(11) is connected to said around plane (14) which is arranged toreactively couple with said ground plane associated with said dielectricsubstrate (20); the arrangement being operable to provide a singleoutput on a dielectric substrate from a coaxial cable input.
 2. Anarrangement according to claim 1, wherein the dielectric substrate ofthe intermediate board is manufactured from PTFE.
 3. An arrangementaccording to claim 1, wherein said first dielectric substrate is apolyester film.
 4. An arrangement according to claim 1, wherein theprinted circuit is arranged in the form of microstrip.
 5. An arrangementaccording to claim 1, wherein said ground plane (15) is printed on theintermediate dielectric.
 6. An arrangement for transferring highfrequency microwave signals between a cable and a printed circuit on adielectric substrate, the arrangement comprising: a coaxial cable (10)having an inner conductor (16) and an outer conductor (11), a firstground plane (14), a first dielectric substrate (20) having a firstprinted circuit, an intermediate dielectric substrate (18) having asecond printed circuit and a ground plane (15);wherein the secondprinted circuit comprises a five port rat-race coupler and two Wilkinsondividers, each Wilkinson divider, having an input and two outputs;wherein the inner conductor of the coaxial cable is connected to a firstport of said five port rat-race-coupler, wherein the third and fourthports of said rat-race-coupler are connected to ground by terminatingresistors (R1, R2); and wherein the second and fifth ports (54, 56)provide output ports which are connected to the input of the Wilkinsondivider which is operable to provide two output ports wherein the outerconductor of the coaxial cable (11) is connected to said ground plane(14) which is arranged to reactively couple with said ground planeassociated with said dielectric substrate (20); the arrangement beingoperable to provide four outputs to a dielectric substrate from acoaxial cable input.
 7. A method of transferring high frequencymicrowave signals between a cable and a printed circuit on a dielectricsubstrate, in an arrangement comprising: a coaxial cable (10) having aninner conductor (16) and an outer conductor (11), a first ground plane(14), a first dielectric substrate (20) having a first printed circuit,an intermediate dielectric substrate (18) having a second printedcircuit and a ground plane (15);wherein the second printed circuitcomprises a five port rat-race coupler, a ground plane (15) and aWilkinson coupler, having two coupling arms and an output; wherein theinner conductor of the coaxial cable is connected to a first port ofsaid five port rat-race-coupler, wherein third and fourth ports of saidrat-race-coupler are connected to ground by terminating resistors (R1,R2); and wherein second and fifth ports (54, 56) are connected to thetwo coupling arms of the Wilkinson coupler which provides a coupledoutput port; wherein the outer conductor of the coaxial cable (11) isconnected to said ground plane (14) which is arranged to reactivelycouple with said ground plane (15) associated with said dielectricsubstrate (18); the method comprising the steps of:transferring saidsignals through an inner conductor (16) of the coaxial cable (10) to thefirst port of the five port rat-race-coupler; transferring the signalsto second and fifth ports (54, 56); transferring the signals from secondand fifth ports (54, 56) to feed, in a balanced fashion, the Wilkinsoncoupler to provide an output at the output port; reactively couplingsignals from the output of the Wilkinson coupler to said dielectricsubstrate (20), whereby a single output from the coaxial cable isestablished on the first dielectric.
 8. An arrangement according toclaim 7, wherein the dielectric substrate of the intermediate board ismanufactured from PTFE.
 9. An arrangement according to claim 7, whereinsaid first dielectric substrate is a polyester film.
 10. An arrangementaccording to claim 7, wherein said ground plane (15) is printed on theintermediate dielectric.
 11. A method of transferring high frequencymicrowave signals between a cable and a printed circuit on a dielectricsubstrate, in an arrangement comprising: a coaxial cable (10) having aninner conductor (16) and an outer conductor (11), a first ground plane(14), a first dielectric substrate (20) having a first printed circuit,an intermediate dielectric substrate (18) having a second printedcircuit and a ground plane (15);wherein the second printed circuitcomprises a five port rat-race coupler and two Wilkinson dividers, eachWilkinson divider, having an input and two outputs; wherein the innerconductor of the coaxial cable is connected to a first port of said fiveport rat-race-coupler, wherein third and fourth ports of saidrat-race-coupler are connected to ground by terminating resistors (R1,R2); and wherein second and fifth ports (54, 56) provide output portsand are each connected to an input arm of a respective Wilkinson dividerto provides four output ports; wherein the outer conductor of thecoaxial cable (11) is connected to said ground plane (14) which isarranged to reactively couple with said ground plane (15) associatedwith said dielectric substrate (18); the method comprising the stepsof:transferring said signals through an inner conductor (16) of thecoaxial cable (10) to the first node of said five port rat-race-coupler;transferring the signals to second and fifth ports (54, 56);transferring the signals from second and fifth ports (54, 56) to feedrespective Wilkinson dividers to provide four outputs; reactivelycoupling signals from the outputs of the Wilkinson dividers to saidfirst dielectric substrate (20), whereby four outputs from the coaxialcable are established on the first dielectric.
 12. An arrangement fortransferring high frequency microwave signals between a cable and aprinted circuit on a dielectric substrate, the arrangement comprising: acoaxial cable (10) having an inner conductor (16) and an outer conductor(11), a first around plane (14), a first dielectric substrate (20)having a first printed circuit, an intermediate dielectric substrate(18) having a second printed circuit and a ground plane (15);wherein thesecond printed circuit comprises a five port rat-race coupler; whereinthe inner conductor of the coaxial cable is connected to a first port ofsaid five port rat-race-coupler which conductor is orientedsubstantially normally to the dielectric substrate; wherein the thirdand fourth ports of said rat-race-coupler are connected to ground byterminating resistors (R1, R2); wherein ground patches are providedeither side of the first port whereby to confine modes of propagation toenable the transfer of signals from an input coaxial connector at thefirst port to the second and fifth ports; wherein the outer conductor ofthe coaxial cable (11) is connected to said ground plane (14) which isarranged to reactively couple with said ground plane (15) associatedwith said dielectric substrate (18); the arrangement being operable toprovide two outputs on said first dielectric substrate (20) coupled viathe second and fifth ports of said rat-race coupler from said coaxialcable (10).
 13. An arrangement according to claim 12, wherein thedielectric substrate of the intermediate board is manufactured fromPTFE.
 14. An arrangement according to claim 12, wherein said firstdielectric substrate is a polyester film.
 15. An arrangement accordingto claim 12, wherein said ground plane (15) is printed on theintermediate dielectric.
 16. A method of transferring high frequencymicrowave signals between a cable and a printed circuit on a dielectricsubstrate, in an arrangement comprising: a coaxial cable (10) having aninner conductor (16) and an outer conductor (11), a first around plane(14), a first dielectric substrate (20) having a first printed circuit,an intermediate dielectric substrate (18) having a second printedcircuit and a ground plane (15);wherein the second printed circuitcomprises a five port rat-race coupler; wherein the inner conductor ofthe coaxial cable is connected to a first port of said five portrat-race-coupler which conductor is oriented substantially normally tothe dielectric substrate; wherein the third and fourth ports of saidrat-race-coupler are connected to ground by terminating resistors (R1,R2); wherein ground patches are provided either side of the first portwhereby to confine modes of propagation to enable the transfer ofsignals from an input coaxial connector at the first port to the secondand fifth ports; wherein the outer conductor of the coaxial cable (11)is connected to said ground plane (14) which is arranged to reactivelycouple with said ground plane (15) associated with said dielectricsubstrate (18); the method comprising the steps of:transferring saidsignals through an inner conductor (16) of the coaxial cable (10) to thefirst port of said five port rat-race-coupler; transferring the signalsto second and fifth ports (54, 56); transferring the signals from secondand fifth ports (54, 56) to provide two outputs; reactively couplingsignals from the two outputs from said rat-race coupler to said firstdielectric substrate (20), whereby to provide two outputs from thecoaxial cable on the first dielectric from the coaxial cable input.